Conventionally, in a molding machine; for example, in an injection-molding machine, resin heated and melted in a heating cylinder is injected under high pressure and charged into a cavity of a mold apparatus, and the injected resin is cooled and solidified in the cavity, whereby a molded product is obtained.
For such a molding operation, the injection-molding machine includes a mold apparatus, a mold-clamping apparatus and an injection apparatus. The mold-clamping apparatus includes a stationary platen and a movable platen. The movable platen is advanced and retreated by means of a mold-clamping cylinder, whereby the mold apparatus is closed, clamped, and opened.
Meanwhile, the injection apparatus, which is generally of an in-line screw type, includes a heating cylinder for heating and melting resin fed from a hopper, and an injection nozzle for injecting the molten resin. A screw is disposed in the heating cylinder in a reciprocative and rotatable condition. When the screw is advanced by means of a drive section connected to the rear end thereof, resin is injected from the injection nozzle. When the screw is retreated by means of the drive section, metering of resin is performed.
FIG. 1 is a cross sectional view showing a main portion of a conventional injection apparatus.
In this drawing, reference numeral 11 denotes a heating cylinder. An injection nozzle 12 is attached to the front end of the heating cylinder 11, and a screw 14 is disposed within the heating cylinder 11 such that the screw 14 can rotate and can advance and retreat. The screw 14 is composed of a flight portion 15 and an injection portion 16, and is connected to an unillustrated drive apparatus via a shaft portion 21 at the rear end. The injection portion 16 is composed of a head portion 41, a rod portion 42 extending rearward from the head portion 41, a check ring 43 disposed around the rod portion 42, and a seal ring 44 attached to the front end of the flight portion 15. Notably, the head portion 41, the rod portion 42, etc. constitute a screw head. Further, the check ring 43 and the seal ring 44 serve as a reverse-flow prevention apparatus for preventing reverse flow of resin during an injection step. The drive apparatus is composed of an injection motor and a metering motor. The flight portion 15 has a cylindrical body of the screw; i.e., a screw body, and a spiral flight 23 formed on the outer circumferential surface of the screw body, so that a spiral groove 24 is formed along the flight 23.
A supply port 25 is formed in the heating cylinder 11 in the vicinity of the rear end thereof, and an unillustrated funnel-shaped hopper is disposed at the supply port 25. Resin in the form of pellets stored in the hopper is supplied to the interior of the heating cylinder 11 via the supply port 25.
The supply port 25 is formed at a location such that the supply port 25 faces a rear end portion of the groove 24 when the screw 14 is positioned at the fowardmost position within the heating cylinder 11; i.e., at the advancement limit position. The flight portion 15 has a supply portion P1, a compression portion P2, and a metering portion P3, formed in this sequence from the rear end to the front end. The supply portion P1 receives the resin supplied via the supply port 25. The compression portion P2 melts the supplied resin while compressing the resin. The metering portion P3 meters a predetermined amount of the molten resin each time.
In the injection apparatus having the above-described configuration, in a metering step, the screw 14 is rotated through drive of the metering motor. Thus, the resin supplied from the hopper into the heating cylinder 11 is caused to advance along the groove 24, and is heated and melted. With this operation, the screw 14 is retreated. Since the check ring 43 moves forward in relation to the rod portion 42 as the screw 14 is retreated, the resin having reached the front end of the flight portion 15 passes through a resin passage between the rod portion 42 and the check ring 43, and reaches a space located forward of the screw head. Accordingly, an amount of molten resin corresponding to a single shot is accumulated forward of the screw head, in a state in which the screw 14 is positioned at the rearwardmost position within the heating cylinder 11; i.e., at the retreat limit position.
Subsequently, in an injection step, the screw 14 is advanced through drive of the injection motor, whereby the resin accumulated forward of the screw head is injected from the injection nozzle 12, and is charged into a cavity of an unillustrated mold apparatus.